Various antenna configurations have been used for simultaneous transmit and receive (STAR) applications with omnidirectional pattern coverage. For example, a ring array antenna having a linear phase progression with increasing angle around the array circumference can be used to produce the omnidirectional radiation pattern. For an even number of antenna elements in the ring array, each opposing pair of antenna elements is fed anti-phase, that is, the two antenna elements differ in phase by 180°, to generate a radiation pattern having a null at the center of the ring array.
Ring array antennas are capable of full duplex operation wherein the antenna can transmit and receive simultaneously in the same frequency band. These ring array antennas have a substantially omnidirectional pattern in the azimuth plane for both transmit and receive operations. The receive antenna includes four antenna elements each having a beamwidth in the azimuth plane that is slightly greater than 90°. The receive antenna elements are arranged symmetrically about a midpoint that lies in the azimuth plane. The receive beams of the four receive antenna elements face outward, that is, away from the midpoint, and together the receive beams cover the full azimuth plane. The transmit antenna is a colinear set of dipole elements that is orthogonal to the azimuth plane and centered on the midpoint of the receive array. A nulling circuit connected to the receive array provides further isolation between transmit and receive operations by imposing a 180° phase difference between geometrically opposite receive antennas. Adjacent antenna elements in the four-element receive array are offset in phase by 90°.
A high-isolation ring array antenna system with collocated antennas and cancellation of coupled signals for simultaneous transmit and receive has been developed. In this system, a vertical transmit dipole antenna is mounted on top of a mast and an array of vertical receive dipole antenna elements is supported on the mast below the transmit vertical dipole antenna. The receive dipole antenna elements are arranged in pairs wherein one of the elements in the pair is located on the opposite side of the mast from the other element in the pair. The receive dipole antenna elements are symmetrically located in the omnidirectional antenna pattern of the transmit dipole antenna. The coupling to each receive dipole antenna element is equal and in-phase with respect to the coupling for each of the other receive dipole antenna elements. The total coupling is effectively zero due to the antiphase combination of signals from the two receive dipole antenna elements in each pair of opposing elements. By reciprocity, cancellation of coupled signals is also achieved when the vertical transmit dipole antenna element is instead used to receive and when the array of receive dipole antenna elements is instead used to transmit. Dipole and monopole high-isolation antenna systems can also be configured on ground planes. For a four-element array, the phasing for a progressive phase variation is 0°, 90°, 180° and 270°. By way of example, measured isolation data on the order of 60 dB for a dipole array antenna system that operates in the 30 to 88 MHz band has been acquired.
A dipole ring array antenna system for generating circularly polarized radiation patterns having a null on axis has been developed. Opposing antenna elements in the ring array are driven so that their electrical phases differ by 180°. For an eight element dipole array, the relative phasing along the circumference of the array is a so-called third mode, that is, the phase variation moving along the ring array is 0°, 225°, 90°, 315°, 180°, 45°, 270° and 135° degrees which yields circular polarization for horizontally-oriented dipole antenna elements.
A ring array of four progressively phased (0°, 90°, 180° and 270°) dipole antenna elements and a central dipole for improved isolation has been studied. In particular, geometries in which the central dipole is at the same height and elevated above the ring array were analyzed. The elevated dipole geometry was shown to increase isolation by 3 dB.